1. Field of the Invention
The present invention relates to an apparatus for controlling an internal combustion engine, and more particularly to an apparatus for controlling a flow control valve disposed in an intake air passage of an internal combustion engine, such as a throttle valve or the like, for converging a given control quantity such as an amount of intake air introduced into the internal combustion engine to a target value therefor according to a sliding mode control process.
2. Description of the Related Art
Apparatus for controlling internal combustion engines control a flow control valve disposed in an intake air passage of an internal combustion engine, such as a throttle valve or the like, for converging various control quantities, e.g., an amount of intake air, a rotational speed, an output torque, etc. relative to the internal combustion engine to desired target values according to a feedback control process.
It is the general practice to employ a PI (proportional plus integral) control process as the feedback control process. However, the PI control process is difficult to achieve control stability against the effect of disturbance or the like, and finds it extremely difficult to establish gain constants relative to proportional and integral terms.
In recent years, there have been proposed apparatus which employ a sliding mode control process for the above feedback control process, as disclosed in Japanese laid-open patent publications Nos. 7-133739 and 8-61122, for example.
In the proposed apparatus, a command signal (so-called control input) corresponding to a command value for a manipulative quantity for the throttle valve for converging an output torque, which is a control quantity of an internal combustion engine, to its target value is generated according to the sliding mode control process, and an actuator of the throttle valve is operated on the basis of the generated command signal.
Generally, the sliding mode control process is more stable against the effect of disturbance or the like than the PI control process. According to the sliding mode control process, it is possible to converge a control quantity, such as an output torque or the like, of an internal combustion engine that can be controlled by the throttle valve, stably to a target value.
The sliding mode control process requires a model of an object to be controlled thereby in order to construct an algorithm for the processing thereof. Heretofore, as described in the above publications, the model is established as a continuous system or, more specifically, a continuous-time system.
For example, according to the disclosure of the above publications, a system including a throttle valve, an actuator thereof, and an internal combustion engine is handled as an object to be controlled, and each of the behavioral characteristics of a system including the throttle valve and the actuator and the behavioral characteristics of the internal combustion engine is expressed as a continuous-system model of a time lag of first order (a differential equation of first order) (the overall object to be controlled is expressed by a differential equation of second order). The algorithm for the processing of the sliding mode control process is constructed on the basis of the continuous-system model.
However, since the object to be controlled by the sliding mode control process has heretofore been expressed as a continuous-system model, the conventional apparatus suffer the following drawbacks:
In the sliding mode control process, it is necessary to establish, in addition to the above model, a linear function referred to as a switching function based on the model (such a linear function is called a sliding line constant in the above publications). When the continuous-system model of the object to be controlled is established, the switching function is defined by a linear function composed of the difference between a control quantity and a target value therefor and a time differential of the difference (a rate of change of the difference). According to the above publications, an input of the object to be controlled, i.e., a command signal corresponding to a command value for the manipulative quantity of a throttle valve, is generated in order to converge the value of the switching function thus defined to "0". In this manner, the difference and its rate of change are controlled to achieve a stable state in which the difference between the control quantity (output torque) of the object to be controlled and its target value, and its rate of change become "0", i.e., the control quantity is steadily converged to its target value.
The conventional apparatus which express the object to be controlled as a continuous-system model require the rate of change (time differential) of the difference between the control quantity and its target value as a component of the switching function that is needed for the processing of the sliding mode control process.
However, inasmuch as the rate of change generally cannot directly be detected easily by a sensor or the like, it is customary to determine the rate of change from the detected value of the control quantity or its predicted value through calculations. Accordingly, the value of the rate of change tends to contain a calculation error. Moreover, in situations where the control quantity tends to contain an instantaneous noise component (such situations are likely to occur with internal combustion engines), the value of the rate of change tends to suffer a large error, i.e., tends to lack reliability.
Though the sliding mode control process has excellent properties, those properties possibly fail to be fully utilized, making it impossible to converge the control quantity stably to the target value.
In the continuous-system model, generally, it is difficult to identify the values of model parameters (such as a time constant in the above publications) which define the actual behaviors of the model. It has been necessary to express the continuous-system model as a discrete system (specifically, a discrete-time system), and to use a known computer-processed algorithm to identify the model parameters of the discrete-system model.